Introduce auto-initializing wrappers for JNI method and field IDs
to JavaHL's jniwrapper.

* subversion/bindings/javahl/native/jniwrapper/jni_env.hpp
  (Java::MethodID, Java::FieldID): New wrapper classes with value smeantics.
* subversion/bindings/javahl/native/jniwrapper/jni_base.cpp,
  subversion/bindings/javahl/native/jniwrapper/jni_list.hpp,
  subversion/bindings/javahl/native/jniwrapper/jni_list.cpp,
  subversion/bindings/javahl/native/jniwrapper/jni_object.hpp:
   Replace jmethodID and jfieldID class members with their respective
   wrapper classes; saves a bit of typing in constructor initializer
   lists.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1540484 13f79535-47bb-0310-9956-ffa450edef68